Hm, a resistor at the gate? I have the 4.5Volt at the LED even with the gate not connected to the Arduino.

To test. disconnect from the Arduino and tie the gate to ground using your pull-down resistor. You can short it to ground if you like. Check if you still have 4.5V

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Source voltage? You mean the 12Volt that go to the LED? The IRLU8726PBF is made for up to 30Volt.

Sorry, that is fine, what I meant was do you have the correct limiting resistor for your LED for the voltage being supplied?

Are your grounds okay? Short, connected to a single point. Is your CAT6 shielded? and if so, is the grounding at a single point - it's not unusual for Radio2 to take control of your circuit If you have wires spanning the house, you will have quite a nice antennae going. Look out for the WOW! signal

Grounding the gate changes nothing.There is no limiting resistor cause its a 12V LED. Its one of those you can swap withthe 12V halogen lamps in halogen systems: http://www.amazon.de/ersetzt-35Watt-Halogen-warmes-Kelvin/dp/B0076J5AHG/They are really bright like 35Watt normal lamps and make a nice light.The first LED i saw that really make sense when it comes to price<->power saving (amortization).

There is no CAT6 at the moment. Everything is short and on the breadboard

But when you talk about shielding. Would you always connect the shield to ground?Both ends ore just one?I use just shielded cable:Stranded CAT.5E SF/UTP for switches and flow sensors.Stranded CAT.6 S/STP for network.Solid CAT.6 S/FTP AWG23 for 12V power to the LEDs and Arduinos.Lappkabel UNITRONIC LiYCY 2x0,25mm2 where i have to spread the 12V: http://products.lappgroup.com/online-catalogue/data-communication-systems/low-frequency-data-transmission-cables/din-colour-code/unitronic-liycy.html

And i want to make a extra cable from the breaker box to my home stereo.I want to use Lappkabel ÖLFLEX 120 CH 3x2,5mm2: http://products.lappgroup.com/online-catalogue/power-and-control-cables/various-applications/halogen-free/oelflex-120-ch.htmlBut i'm not sure what to to with the shield on this 220Volt cable.

I'm no expert, but since MR16 lamps are designed to be compatible with halogen lamps their internal circuitry, by design, has to work in concert with transformers and dimmers made for halogen lamps. This compatibility requirement has led to proprietary solutions which may have important side-effects.

If i connect the LED directly to the 12 Volt power supply i have 12 Volt at the LED.In the PWM circuit i have the funniest volts.

There are allot PWM circuits out there but PWM is what the Arduino does.Maybe a LED driver?I found this: http://www.led-treiber.de/html/leds_grundlagen.html#PWM-Dimmerand they write it works with all there LED drivers: http://www.led-treiber.de/html/getaktete_treiber.html

How about a power supply for all the LEDs and a "reverse phase control dimmer" in front of themwho is controllable with the Arduino?

But i think its there cause there are to many factors with transformers and dimmers.I have a 12 volt halogen electronic transformer that is dim able with "reverse phase control dimmer" and"phase control dimmer". But here the dimmer is in front of the transformer and its AC.I was thinking DC and PWM the whole time - maybe there is another way.I mean if i feed less volt to the LED its darker.Maybe AC instead of DC...

Edit:Just found this: http://www.youtube.com/watch?v=XgwNyO01mIg

So i need to do more research and use other words in google.Was looking the whole time for PWM stuff.

After thinking i cant open the LED without breaking it i have it open now.The 3 LEDs are series-connected.The electronic was sealed in some silicon.Looks close to this ones: http://detail.china.alibaba.com/pic/914205741.html

Its a MT7201 1A LED driver: http://www.maxictech.com/en/UploadFiles/MT7201_rev1.1_EN.pdfAnd the circuit is close to this one: http://www.led-treiber.de/html/getaktete_treiber.html#MIC3201But there are some diodes in front of it as rectifier.

Looks like you can PWM control this driver.I will try to reverse engineer the circuit/PCB to find out what the LEDs need.

If i build my own driver, is it better to1. put the MOSFET and driver near the LED2. put the MOSFET near the Arduino and the driver near the LED3. put the MOSFET and the driver near to the Arduino

And sure there are dimable LEDs out there, but they are not cheap and this have a reallygood Colour Rendering Index and Color temperature.

Ok, i did some measuring as you can see in the attachment.I cant figure out the driver circuit cause its to small.But there is a R25 and 3R00 on the PCB.If i look at the M7201 data sheet that would mean they drive the LEDs with 0.1 / 0,25? = 400mAThat is close to my measurement.If the 0.25? and 3? resistors are parallel its right: 0.1 / 0.23? = 430mA

If i look at the CRI and Color temperature and everything i bet they are Cree XLamp XP-E High-Efficiency White LEDs. http://www.rapidonline.com/pdf/55-2568.pdfBut if the forward voltage is 3V, why do i measure 9V?Shouldn't it be: 12V - 3 x 3V = 3V

So from all the reading around it looks like the driver needs to be near the LEDs.So i need a third wire for PWM?But how about the PWM cable length?

yea you need to connect pwm output to the adj pin as suggested on page 8-9 of the datasheet.i dont think cable length does matter, as long its not something like half a meter. well you can calculate loss of voltage depending on cable length.

0.5 Meter?It's a house. There are 5-10 meters...I found this driver: http://www.leds.de/out/media/KSQ.pdfAnd they write to make the cable from the supply not longer then 1,5 meters.Or is it that they assume that power cables are always unshielded?

@lax123, i thank you that you want to help me. But that is basic stuff i know.

And this calculator shows what i don't understand. Why so big and short cables for LED's?I read stuff like 0.75mm2 - 1.5mm2 on most sides.But if you look at eldoLED POWERdrive they write 3 meters to the LEDs.And they write 0.2mm2 per 1 ampere.They also wrote somewhere that 6 meters is no problem with CAT cable and also wrote thatthey have customers who made up to 50 meters.

What i can't find is a answerer to PWM over longer distance.If i have a LED driver with PWM in - how long can i make the PWM line from the Arfuinoto the driver.

There is also the option to dim analog. This would change the color temperature a little.But i can't see in the Cree datasheets what color temperature at which ampere.They write color temperatures from 2600 to 4300 for the XP-E High-Efficiency White LEDs.

Damn is this LED stuff complicated when it comes to dimming.I wish i had a oscilloscope